Students attended graduation at the City College of New York last year. The federal Education Department has raised the possibility that students’ acceptance into a loan forgiveness program could be rescinded.CreditCreditSpencer Platt/Getty Images

More than 550,000 people have signed up for a federal program that promises to repay their remaining student loans after they work 10 years in a public service job.

But now, some of those workers are left to wonder if the government will hold up its end of the bargain — or leave them stuck with thousands of dollars in debt that they thought would be eliminated.

In a legal filing submitted last week, the Education Department suggested that borrowers could not rely on the program’s administrator to say accurately whether they qualify for debt forgiveness. The thousands of approval letters that have been sent by the administrator, FedLoan Servicing, are not binding and can be rescinded at any time, the agency said.

The filing adds to questions and concerns about the program just as the first potential beneficiaries reach the end of their 10-year commitment — and the clocks start ticking on the remainder of their debts.

Four borrowers and the American Bar Association have filed a suit in United States District Court in Washington against the department.

The plaintiffs held jobs that they initially were told qualified them for debt forgiveness, only to later have that decision reversed — with no evident way to appeal, they say. The suit seeks to have their eligibility for the forgiveness program restored.

“It’s been really perplexing,” said Jamie Rudert, one of the plaintiffs. “I’ve never gotten a straight answer or an explanation from FedLoan about what happened, and the Department of Education isn’t willing to provide any information.”

The forgiveness program offers major benefits for borrowers, advocates say, to the point of persuading some people to take public service jobs instead of more lucrative work in the private sector. The program generally covers people with federal student loans who work for 10 years at a government or nonprofit organization, a diverse group that includes public school employees, museum workers, doctors at public hospitals and firefighters. The federal government approved the program in 2007 in a sweeping, bipartisan bill.

About 25 percent of the nation’s work force may qualify for the program, the Consumer Financial Protection Bureau estimated. Eligibility is based on a borrower’s employer and whether it meets the program’s rules, not on the specific work an applicant does.

On its website, the Education Department directs borrowers who believe their employer qualifies to submit a certification form to FedLoan. If the form is approved, the Education Department transfers the borrower’s loans to FedLoan, which collects payments and tracks the borrower’s progress toward the 120 qualifying monthly payments they must make before the remaining balances will be forgiven.

Only a small fraction of the millions of workers who might qualify for the program have begun the process of using it. Fewer than 553,000 borrowers have submitted at least one certification form to FedLoan and received its approval, according to Education Department data. Borrowers are encouraged to submit a new certification form each year.

But some of those approved borrowers might get bad news because it is unclear whether the certifications are valid.

Mr. Rudert submitted the certification form in 2012 and received a letter from FedLoan affirming that his work as a lawyer at Vietnam Veterans of America, a nonprofit aid group, qualified him for the forgiveness program. But in 2016, after submitting his latest annual recertification note to FedLoan, he got a denial note.

The decision was retroactive, he was told. None of his previous work for the group would be considered valid for the loan forgiveness program.

What changed? Mr. Rudert said he did not know. After filing a complaint with the Consumer Financial Protection Bureau, he received a reply from FedLoan saying that his application “had initially been approved in error.” He has not been told what the error was, and has not found any way to appeal the decision.

Mr. Rudert and the American Bar Association filed their suit in December, alleging that the Education Department acted “arbitrarily and capriciously” in making its decisions about which employers qualified.

Last week, the department filed a reply that said that FedLoan’s responses to borrowers’ certification forms cannot be trusted.

A FedLoan approval letter “does not reflect a final agency action on the borrower’s qualifications” for the forgiveness program, the department wrote.

The idea that approvals can be reversed at any time, with no explanation, is chilling for borrowers. Mr. Rudert, who graduated from law school owing nearly $135,000 on student loans, said he would have picked a different employer if he had known that his work at Vietnam Veterans of America would not qualify.

A FedLoan spokesman would not comment on the case, referring questions to the Department of Education. A department spokesman also declined to comment on the suit or on any of the issues it raised, including whether any mechanism exists for borrowers to challenge a denial.

That lack of transparency has been a hallmark of the forgiveness program, said Natalia Abrams, the executive director of Student Debt Crisis, an advocacy group.

The program’s rules are complex. Only certain types of federal loans qualify, meaning that many borrowers need to restructure their debt to make it eligible — and the Education Department has done little to clarify gray areas, Ms. Abrams said.

No borrowers’ debts have been eliminated. Because 10 years of service are required, the first wave of qualified workers will be eligible to submit applications for debt forgiveness in October.

At that point, others whose certifications were approved by FedLoan could discover that the Education Department has a different position. Some employers clearly qualify — the definition of a “government organization” is fairly straightforward — but the rules for certain nonprofit organizations are harder to interpret.

“It’s kind of a no man’s land,” Ms. Abrams said. “We don’t know how this will pan out.”

Linda Klein, president of the American Bar Association, called the department’s response “illogical, untenable and bewildering.” An unreliable certification system “exposes those undertaking public service work — exactly what Congress intended them to do — to crippling financial risk,” she said.

Mr. Rudert left Vietnam Veterans of America in 2015 and now works at Paralyzed Veterans of America, helping former service members appeal denied applications for disability benefits.

The work is almost identical to what he did in his former job, Mr. Rudert said. Last year, FedLoan approved his certification request and deemed Paralyzed Veterans of America a qualified employer.

A version of this article appears in print on , on Page B1 of the New York edition with the headline: U.S. Promises to Clear Debt for Students May Be False. Order Reprints | Today’s Paper | Subscribe